Rotary actuators are driven by voice/coil motors (VCMs) in which electrical impulses are translated into movement of transducers or head elements. This type of actuator has found wide usage in modern disk drive recording systems. Typically, the VCMs operate in conjunction with servo control systems to position the servo and read/write head elements radially over the spinning magnetic disk.
VCM type actuators consist of a permanent magnetic structure and a moveable coil or bobbin attached to a comb of arms carrying the read/write heads. The coil is positioned either between or around a pair of permanent magnets which are housed within a steel structure. FIG. 1 illustrates a cross-section of a typical prior art actuator structure. Steel support structure 10 is used for energizing the coils attached to the actuator arm assembly. Structure 10 includes respective upper and lower poles 13 and 14 and centerpole 15. Magnets 11a and 11b are shown attached to plates 13 and 14, respectively, within support structure 10. These magnets are suitably polarized (i.e., north pole directed towards centerpole 15) so as to develop magnetic lines of flux which extend across air gaps 12a and 12b. The moveable coil is wrapped around centerpole 15 through gaps 12a and 12b.
According to the principles of physics, passing a current through a coil placed in a magnetic field results in a force upon the coil which can then be translated into a movement of the servo and read/write heads about a pivot axis. Movement of the heads is controlled by means of ordinary servo-control techniques which position the heads directly over various tracks of the magnetic recording medium, i.e., the rotating magnetic disk.
As is appreciated by practitioners in the field, one of the key performance parameters for disk files is access time; that is, the time it takes the heads to move from one track to another in response to coil current. Access time is directly related to the response time of the coil (i.e., the rise time or time constant) to an applied current.
FIG. 2 shows a plot of coil current versus response time for various prior art actuator designs. In each of these plots, the coil current rises an exponential function of time. Generally, for optimal performance it is desired to have a response curve such as that depicted by line 20 in FIG. 2. Line 20 represents the coil response for the structure of FIG. 1 with centerpole 15 removed. In other words, line 20 represents the response of a coil wrapped in air, as opposed to one wrapped around a steel centerpole member. While the response curve for a support structure without a centerpole (i.e., air) is desirable for optimal access times, it suffers from the drawback of very low flux density.
Without centerpole 15, the flux density across gap 12a and 12b is substantially lowered. This translates into a lower torque constant for the actuator and a bigger variation in magnetic air gap density. So in effect, in the absence of centerpole 15, a fast rise time can be realized, but at the expense of a lower, non-linear torque constant. For this reason, virtually all of the support structures employed in disk drive systems include some kind of a centerpole member.
As stated above, incorporating a centerpole into structure 10 greatly increases the flux density. However, because of the self-magnetization effect wherein the field strength in the center region is dependent upon the permeability there; the response of an actuator having a centerpole is generally greatly diminished. Such a condition is shown by dashed line 21 in the plot of FIG. 2. Effectively, the presence of centerpole 15 increases the inductance of the coil, thereby slowing the coil's ability to move in response to an applied current.
In an attempt to circumvent this problem, the prior art wrapped a copper conductor around centerpole 15, either in a one piece extrusion or as multiple pieces soldered together. This type of actuator is commonly referred to as a shorted copper turn. Normally the shorted turn was stationary. The purpose of the shorted turn was to lower the effective inductance and to decrease the effective time constant so that a quicker current response could be achieved. However, this type of design is also prone to certain problems.
One of the drawbacks of the shorted copper turn design is the fact that the steel-to-magnet gap must be increased to accommodate the additional copper material. This increase in the steel-to-magnet gap lowers the torque available per unit of coil current. Another more fundamental problem, is the fact that the shorted copper turn provides a response time which is excessively fast--actually jerking the actuator so hard as to generate a very high rate of change of acceleration, and thus generate mechanical resonances within the system. Moreover, the shorted copper turn approach tends to complicate the design of the control loop servo tracking mechanism since the response curve for the shorted copper turn design includes two to three time constants.
Dashed line 22 of FIG. 2 illustrates a typical response for a centerpole design with a shorted copper turn. As is clearly seen, the response of this type of structure is governed by two distinct time constraints.
Yet another prior art approach provided for the incorporation of a copper bar directly into centerpole 15. This approach is exemplified in U.S. Pat. No. 4,652,779. Although this design does not require an increase in the steel to magnetic gap, it is not without its drawbacks. As discussed earlier, the use of copper decreases the response time beyond the point of optimization such that mechanical resonances and vibrations are frequently generated within the system. Furthermore, the cost of the copper material and the associated labor cost for consolidating the copper bar into the centerpole is excessive for certain designs. The accumulated manufacturing tolerances are also high due to the copper thickness and the epoxy glue typically used to bond the parts together. Also, nickel plating is sometimes required to protect the copper surface from corrosion. Thus, designs which employ a copper bar integrated into the centerpole are often costly and require complex manufacturing.
What is needed then is a actuator support structure which provides a response curve which approaches the ideal of a moving coil in air. But at the same time the support structure should provide a relatively high flux density to maintain a linear torque constant. As will be seen, the present invention provides a solution to this problem by means of optimally sized and spaced openings disposed in the centerpole of the support structure. For reasons which will be explained in more detail shortly, the present invention provides improved access times with higher flux densities when compared with prior art designs.